Most grippers available nowadays are designed for a single task. They are usually simple, robust and cheap. Unfortunately they are not very flexible and can, most of the time, only grasp few types of objects. These grippers are commonly used in industrial applications for grasping tasks. They have only few degrees of freedom (DOF). In fact, most of them have one DOF and one actuator.
Other grippers are designed to be more flexible and are able to grasp a large variety of objects. Moreover, most of these types of grippers are not only able to grasp objects but can also manipulate them, as does the human hand.
Unfortunately, these grippers (or mechanical hands) are more complex, fragile, apply small grip forces compared to conventional grippers, costly and difficult to control and integrate to a system. These grippers have several DOF and actuators and are even sometimes equipped with tactile sensors.
Finally, other grippers are designed to merge the advantages of the latter two cases. This is possible by using a mechanical concept called underactuation. An underactuated gripper is a gripper that has less actuator than DOF. The basic concepts of underactuation are presented in more details in Hirose et al. (Hirose, S., Umetani, Y.: The development of soft gripper for the versatile robot hand. Mechanism and Machine Theory 13, 351-358 (1978)) and in Shimojima et al. (Shimojima, H., Yamamoto, K., Kawakita, K.: A study of grippers with multiple degrees of mobility, JSME International Journal 30(261), 515-522 (1987)). This leads to grippers that are flexible, robust and powerful without the disadvantages of having several actuators (cost and complexity).
Underactuation can be achieved by using tendons such as disclosed in U.S. Pat. No. 3,694,021 to Mullen, U.S. Pat. No. 5,200,679 to Graham and U.S. Pat. No. 5,080,681 to Erb. Typically, the tendon-based mechanisms can provide less force to the load and suffer from friction and compliance problems. Hence, they are less suitable for industrial applications requiring large grip force or repeatability.
Underactuation can further be achieved using mechanisms such as bars or gears. Underactuated fingers using bars or gears have been made with two phalanges such as disclosed in U.S. Pat. No. 3,927,424 to Itoh or with three phalanges such as disclosed in U.S. Pat. No. 4,834,443 to Crowder et al. Even though it is possible to have a plurality of phalanges, it is not a popular design since it leads to a much more complex design without adding significant versatility.
The above mentioned references disclose underactuated fingers that are capable of providing an encompassing grasp or an enveloping grasp of an object. However, these references do not disclose underactuated fingers that are capable of providing a so called pinch grasp where an object is picked up using only distal phalanges of at least two fingers. To accomplish this pinch grasp, the distal phalanges must maintain a parallel orientation with respect to each other as they travel toward each other for grasping an object in a pinching manner just as with industrial parallel jaw grippers. This type of grasping is very important in the industry, since it ensures a better precision on the positioning of the object than with the encompassing grasp. Also, many types of objects or situations require to use the pinch grasp. One can think for instance at a small object lying on a desk where the pinch grasp is the only suitable way of taking it.
For doing the pinch grasp, the distal phalanges must stay parallel to each other automatically and without the use of an algorithm. Such a feature was first proposed in U.S. Pat. No. 5,108,140 to Bartholet. This reference discloses a gripper having underactuated fingers with two phalanges. A cam mechanism is integrated in a double parallelogram for allowing the gripper to provide both pinch and encompassing grasps. Unfortunately such a cam mechanism is complicated to fabricate and imprecise.
A similar solution is proposed in U.S. Pat. No. 5,762,390 to Gosselin et al. There is disclosed a gripper with fingers with three phalanges using an additional series of bars arranged in a parallelogram fashion. Again, the gripper is complex and requires an extensive number of parts thereby negatively affecting the robustness of the gripper and the cost of fabrication.
In US Patent Publication 2010/0181792 to Birglen, there is disclosed a gripper that has fingers with at least three phalanges. In order to avoid using an additional series of bars, each finger has triggered elements between the first two phalanges. Although the complexity of this mechanism is decreased in comparison to the solution proposed by Gosselin et al., it is sensitive to interference between the phalanges and the transmission linkages.
Further disclosed in the Birglen reference, there is a simplified gripper having two-phalanx fingers. As presented in FIG. 23 of the publication, there is the gripper for providing a pinch preshaping. The gripper has a five-bar mechanism that includes the finger, its base and the transmission linkage. The same inventor also published a paper on the subject in L. Birglen, “The kinematic preshaping of triggered self-adaptative linkage-driven robotic fingers”, published in Mechanical Sciences, Vol. 2, pp. 41-49, 2011. The study presented in this paper relies on triggered elements using a spring and a mechanical limit situated on a same joint. To obtain a pinch preshaping, one of the joint requires being locked during the closing sequence of the finger, leaving the phalanges to follow a 4-bar motion. The length of the bars is therefore studied to obtain a parallelogram and ensure that the distal phalanx is kept perpendicular to the palm of the robotic hand until a contact occurs. When this contact is established, the actuation torque will overcome the preloading of the triggered element and initiate the closing of the other phalanx. However when this contact is established above the equilibrium point, the actuation torque will overcome the preloading of the triggered element and initiate the opening of the distal phalanx, thereby possibly causing a contacted object to be ejected.
According to Birglen, the desired pinch grasp is made and is maintained if the contact with the object to be grasped occurs at a particular location on the distal phalanx, called equilibrium point. For a linear contact, Birglen states that the pinch is stable if and only if the location of the equilibrium point is located between both vertices of the line, as shown in FIG. 1. In FIG. 1, Birglen illustrates the geometric behavior of the gripper depending on the position of a contacting object 12 with respect to the equilibrium point (14A, 14B and 14C) of the gripper 10. There is illustrated that the object 12 contacting the distal phalanx of the gripper above the equilibrium point 14A or below the equilibrium point 14C renders an unstable geometry of the gripper. Stability of the gripper geometry can only be achieved when the object 12 contacts the distal phalanx of the gripper right on the equilibrium point 14B. As a pinch grasp can only be provided when the gripper geometry is stable, with Birglen's gripper the object must contact the precise equilibrium point location to provide a pinch grasp.
The objective of the study in Birglen's paper is to maximize the value of the equilibrium point (14A, 14B and 14C). In other words, Birglen tries to obtain an equilibrium point that is as close as possible to the distal end 16 of the distal phalanx. In fact, Birglen assumes that the linear contact is long enough to exceed the tip of the distal phalanx, so an equilibrium point situated near the end of the phalanx will most likely be located between both vertices of the line.
While the maximization of the equilibrium location done by Birglen guarantees the finger to be always stable, the resulting behavior is to accomplish encompassing grasp for most of the contact situations, since a contact made under the equilibrium point leads to an encompassing grasp. As mentioned above, the triggered element in Birglen is used for maintaining the distal phalanx in a perpendicular orientation with respect to the palm before contacting an object. Once a contact occurs, there is disclosed that the actuation torque overcomes the preloading of the triggered element and the motion of the second phalanx is dependent on the position of the contact with respect to the equilibrium point of the finger.
As explained above, the pinch grasp is very important in the industrial field to ensure the precision of the placement and is even necessary to pick certain types of objects, such as small parts. The finger disclosed by Birglen is only capable of providing a pinch grasp when a contact occurs at a very precise location that is the equilibrium point of the finger. When a contact occurs within a portion of the finger that is right below or right above the equilibrium point, the pinch grasp cannot be provided.
In addition to the underactuation between the phalanges of a finger, it is also possible to obtain underactuation between the fingers of a same hand. This will further decrease the number of actuators while maintaining the same number of degrees of freedom. This principle has been disclosed for the actuation of many fingers, for example in U.S. Pat. No. 5,378,033 to Guo et al., and in the literature, see for example the article by G. Guo, X. Qian and W. A. Gruver, “A Single-DOF Multi-Function Prosthetic Hand Mechanism with an Automatically Variable Speed Transmission”, published in the Proceedings of the ASME Mechanisms Conference, Phoenix, Vol. DE-45, pp. 149-154, 1992, and the article by M. Rakik, “Multifingered Robot Hand with Selfadaptability”, published in Robotics and Computer-Integrated Manufacturing, Vol. 5, No. 2-3, pp. 269-276, 1989. In these references, each of the fingers has only one degree of freedom, i.e. the motion of the phalanges is coupled.
Gosselin et al. in turn discloses an underactuation of the phalanges of a finger in combination with an underactuation of the fingers of a hand. For convenience, this principle is termed hyperunderactuation.
In U.S. Pat. No. 3,901,547 to Skinner II and in Guo et al. there is disclosed a gripper having a coupling with gears or grooves for changing the orientation of fingers with respect to one another with only one actuator. The motion of each finger about an axis perpendicular to the palm of the mechanical hand is actuated with only one actuator by coupling their orientation. This is possible through the use of a four-bar mechanisms that connects the base of the fingers, thereby decreasing the number of degrees of actuation and freedom of the system.